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How the universe is accelerating

The 2011 Physics Nobel Prize has been awarded to three cosmologists for discovering – unexpectedly – that the expansion rate of the universe is accelerating. Saul Perlmutter of the Lawrence Berkeley National Laboratory in California won half the prize while the other half was shared between Brian Schmidt of the Australian National University in Weston Creek, Australian Capital Territory, and Adam Riess of John Hopkins University in Baltimore, Maryland.

The history of cosmology is full of twists and turns. Physicists have, for a century, known that the universe is expanding. We now know that this expansion is getting faster and faster: the universe will be getting bigger at a faster rate tomorrow than it is today.

In 1929, Edwin Hubble observed distant galaxies and found that the light emitted from these galaxies was slightly redder than expected. Furthermore, the further away the galaxies were the redder their light appeared. Hubble suggested that the galaxies were hurtling away from us with a velocity that increased with distance. In other words, the universe is expanding.

In fact, this phenomenon is a common everyday experience. For example, when an ambulance drives past you then the siren appears to get higher in pitch as the ambulance moves towards you and then lower in pitch as the ambulance drives away from you.

A similar phenomenon occurs when astronomers look at light from far-off galaxies. Light from a distant galaxy is stretched by the expansion of the universe. This makes the wavelength (the distance from one crest to the other) of light we receive much longer. Since red light occurs towards the longer wavelength end of the spectrum then the light we observe appears redder. In contrast, if the universe were contracting then the wavelength of light from distant galaxies would be squashed and the light would appear towards the blue end of the spectrum. Hubble therefore deduced that the universe is expanding.

Flash forward to 1988: physicists expected that the expansion of the universe ought to be decelerating due to the gravitational pull of matter in the universe. At this time, Saul Perlmutter initiated the Supernova Cosmology Project (SCP) with the aim of measuring this deceleration.

Perlmutter looked at distant type 1a supernovae, the explosive remnants of dense stars known as white dwarfs. Type 1a supernovae are important because they all emit light with a similar brightness. This means that astronomers know the absolute brightness of the supernova. They also know the apparent brightness, as viewed from Earth. (Since objects that are far away are less bright than objects that are close by then the absolute and apparent brightness are not the same.) This means that astronomers can calculate how far away from us these type 1a supernovae are. The redshift of the type 1a supernovae then also gives physicists their velocity.

Perlmutter set about analysing the light from these supernovae expecting to discover by how much the expansion of the universe was slowing down. A few years later, encouraged by Perlmutter’s success in tracking down and observing these supernovae, Brian Schmidt began a similar program; much of the analysis for this was done by Adam Riess, then a post-doc at the University of California, Berkeley .

By 1998, both groups published papers suggesting that the expansion of the universe was accelerating. The fact that both teams came to the same conclusions independently is one reason for how easily the scientific community accepted their startling results.

“It seemed too crazy to be right,” said Schmidt, speaking to the Nobel Committee press conference. However, the result is now firmly confirmed as one of the milestones of twentieth century cosmology. It sits alongside Hubble’s original discovery of the expansion of the universe as well as the discovery of the radiation left over by the Big Bang. This data tells physicists about the early history of the universe. However, the expansion history helps physicists understand the universe’s evolution and, perhaps, its ultimate fate.

But what causes the universe to accelerate? In order to explain the results then even empty space contains an unknown dark energy. This provides an outward pressure that accelerates the expansion of the universe. However, in order for the expansion to be accelerating at the rate it is, about 73% of the universe must be accounted for by this strange substance. Moreover, physicists also know that about 23% of the universe is made up of an invisible dark matter. This means only about 4% of the universe is visible.

“It will be a long time before theorists understand this force,” says Martin Rees, the Astronomer Royal. “It is part of the bedrock nature of space and time.”

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